Synthetic fibers are commonly used as reinforcing materials for rubber structures such as tires, belts, hoses, etc. The synthetic fibers used include polyester fibers such as polyethylene terephthalate fibers, polyamide fibers such as typically nylon 6 fibers and nylon 66 fibers, aromatic polyamide fibers, polyvinyl alcohol fibers, etc. However, since such synthetic fibers have poor adhesiveness to rubber, various means have heretofore been proposed of improving the adhesiveness of synthetic fibers to rubber by coating the surfaces of synthetic fibers with an adhesive agent comprising an epoxy compound, an isocyanate compound, a halogenated phenol compound or the like.
An example of a typical adhesive agent for polyester fibers is a mixture prepared by mixing a reaction product of a halogenated phenol such as 2,6-bis(2',4'-dihydroxyphenylmethyl)-4-chlorophenol and resorcinol-formaldehyde with a liquid comprising a precondensate of resorcinol-formaldehyde and rubber latex (RFL) (see Japanese Patent Publication No. 46-11251). However, a fairly large amount of this adhesive agent must be applied to polyester fibers, and the fibers must be heat-treated at high temperatures in order to provide sufficient adhesive strength (drawing strength). The fibers treated under such severe conditions exhibit an increased initial adhesive strength (drawing strength) in ordinary temperature conditions, but the adhesion of rubber to the cleaved fiber plies is retarded.
A two-step method of applying adhesive agents to polyester fibers is known, in which polyesters are treated with a first treating liquid comprising a mixture of an epoxy compound, a blocked isocyanate and bentonite and then treated with a second treating liquid comprising RFL and an ethylene-urea compound (for example, see Japanese Patent Publication No. 57-29586). However, this method results in polyester fibers having adhesive strength (drawing strength) with low heat resistance. Additionally, the stability of the first treating liquid is poor since the bentonite in the liquid has a high true specific gravity and a large particle size, and therefore is not viscous and is easily precipitated.
Another method is known in which polyester fibers are treated with a first treating liquid comprising a mixture of an epoxy compound, a blocked isocyanate and a latex, and then treated with a second treating liquid comprising RFL (for example, see Japanese Patent Publication No. 60-24226). The polyester fibers treated by this method have a relatively high initial adhesive strength (drawing strength) and the adhesion of rubber to the cleaved fiber plies is enhanced. However, the treated polyester fiber cords become hard and the mechanical strength of the cords is lowered, whereby not only is the difficulty in shaping and working the cords increased, but also the fatigue resistance of the cords is lowered.
As typical examples of means of improving the adhesiveness of polyamide fibers to rubber, there is known a method for producing ultra-high-strength nylon tire-reinforcing cords which involves dipping tire-reinforcing cords comprising ultra-high-strength nylon 6 or nylon 66 fibers having a single yarn denier of 4.5 d or less and a raw yarn strength of 12 g/d or more, in a liquid comprising a resorcinol-formaldehyde precondensate and a rubber latex, while the cord strength is controlled to be 0.5 g/d or more (see Japanese Patent Laid-Open No. 63-175179), and also a method for producing rubber-reinforcing polyamide fibers by heat-treating twisted cords comprising nylon 66 fibers having a raw yarn strength of 12 g/d or more under tension at particular temperatures, followed by dipping them in a dipping liquid comprising a vinylpyridine-styrene-butadiene copolymer rubber latex where the proportion of vinylpyridine is specifically defined relative to the total weight of the latex, whereby the decrease in the strength of the thus-treated cords is prevented (see Japanese Patent Laid-Open No. 1-174628). However, the rubber-reinforcing polyamide fibers obtained according to these methods have various problems inhibiting their practical use: the fibers require a single yarn denier of 4.5 d or less, dipping steps must be applied to the fibers, the improvement in the adhesiveness of the fibers to rubber is still unsatisfactory, and the use of the treated fiber cords is limited.
One example of known techniques for improving the adhesiveness of aromatic polyamide fibers to rubber involves dipping an aromatic polyamide fiber material in a first treating liquid comprising a polyepoxy compound having a hydroxyl group(s) and an epoxy compound having unsaturated bond(s), followed by a heat treatment and thereafter a treatment with a second treating liquid of a resorcinol-formaldehyde-rubber latex (RFL) (see Japanese Patent Laid-Open No. 61-126142). However, the adhesiveness between rubber and the aromatic polyamide fibers obtained is still inferior to that between nylon fibers or polyester fibers and rubber.
Another known method involves treating aromatic polyamide fibers with a first treating liquid comprising a polyepoxy compound, a blocked isocyanate compound and a rubber latex compound, followed by treatment with a second treating liquid comprising a resorcinol-formaldehyde-rubber latex (RFL) and a particular chlorophenol compound (see Japanese Patent Laid-Open No. 3-40875). The aromatic polyamide fibers obtained by this method exhibit adhesiveness to rubber comparable to the adhesiveness of nylon or polyester fibers to rubber, but the method still requires that large amounts of the adhesive compositions be applied to the fibers in order to attain satisfactory adhesiveness. Therefore, the costs associated with the adhesive compositions are high.